Oligomerization of Olefins

Abstract

This article reviews recent studies on oligomerization of olefins catalyzed by transition metal complexes. Ni, Pd, and Fe complexes, having a ligand with a similar structure to the ethylene polymerization catalyst but with less bulky substituents, convert ethylene to the oligomers as a mixture with various chain lengths with Schulz–Flory molecular weight distribution. Cossee-type insertion of ethylene into the M–C bond and frequent elimination of α-olefins are proposed as the major reaction mechanism. The reaction using the Fe catalyst for polymerization and large excess of chain transfer reagents such as ZnEt₂ can yield the oligomers with Poisson distribution. Cr complexes with various ligands promote selective trimerization and/or tetramerization of ethylene to produce 1-hexene and/or 1-octene. The mechanism involving metallacycle is proposed to account for the selectivity. Several Ti and Ta complexes are also effective for the trimerization of ethylene. Oligomerization of α-olefins has been also studied, although the product is frequently composed of branched oligomers and/or inner olefins.

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5.1 Introduction

α-Olefins, with a C=C terminal double bond (CH=CH₂ group), are extensively used as the starting materials for organic and polymer compounds. For example, α-olefins with C4–C8 are used as the comonomer for polyolefins, C10 is for lubricants, and C12–C16 are for surfactants. The α-olefins had been synthesized by Fischer-Tropsch synthesis or cracking of paraffin waxes. Oligomerization of olefins is more recent approach for α-olefins, which has become common and important in these several decades [1–12]. It is worth noting that the olefin oligomerization has advantage in atom economy, low energy cost, and production of the oligomer with even number of the carbon chain. Ethylene oligomerization catalyzed by Ni catalysts has been the most important oligomerization as the industrialized process [1, 2, 5]. The reaction occurs smoothly under mild conditions, but yields a mixture of ethylene oligomers with varied chain lengths. Studies on new catalysts enabled selective oligomerization of ethylene and olefins. This chapter includes recent extension of the former as well as remarkable progress of the latter.

5.2 Oligomerization of Ethylene

5.2.1 General Aspect

Metal-catalyzed polymerization and oligomerization of ethylene are generally composed of four reactions, Initiation, Propagation, Termination, and Chain transfer. Scheme 5.1 summarizes a series of the fundamental reactions. k ₁ and k ₂ stand for the kinetic constants of propagation and of termination and/or chain transfer reaction, respectively. The chain length of the product is dependent on the relative ratio between k ₁ and k ₂. The insertion rate k ₁[ethylene], being much larger than k ₂, produces high molecular weight polymer. In contrast, if k ₁[ethylene] is comparable to k ₂, the dimer should be formed as the major product. The competing rates of insertion and β-hydrogen elimination produce a mixture of oligoethylenes with various lengths. Distribution of the chain length of the formed oligoethylenes mostly obeys the following equation (Schulz–Flory distribution) [13, 14].

Scheme 5.1

Fundamental reactions in ethylene oligomerization

W _m =:

m α^m−1(1−α)²

W _m =:

weight fraction of m mer of oligoethylene

α =:

rate of propagation/(rate of propagation + rate of chain transfer)

k ₁/(k ₁ + k ₂)

The value α is frequently used to evaluate the product with Schulz–Flory distribution, and is obtained empirically from slope of the plot of log(W _m /m) versus m [15, 16]. For example, dimer is preferentially formed when α is close to zero. The following values β and K are also used for evaluation of the reaction.

$$ \beta = {\text{rate of chain transfer/rate of propagation}} = \left( { 1- \alpha } \right) /\alpha \,{=}\,k_{ 2} /\left( {k_{ 1} + k_{ 2} } \right) $$

$$ K = {\text{mol fraction C}}_{{{\text{n}}\, + \,2}}\; {\text{olefin/mol fraction C}}_{\text{n}}\; {\text{olefin }}\left( {\text{chain growth factor}} \right) $$

5.2.2 Ethylene Oligomerization to Olefins with Schulz–Flory Distribution and Dimerization

Earlier works on ethylene oligomerization have been reviewed by Skupińska [1]. Ziegler–Natta type catalysts (Ti, Zr) and Ni-based catalysts were extensively used for the oligomerization because of their high activity and selectivity. Especially, Ni ylide complex (Chart 5.1 1a) is highly active for oligomerization of ethylene and applied industrially to Shell Higher Olefin Process (SHOP). Recently, Matt et al. conducted the oligomerization using the ligand with electron-withdrawing COOEt and CF₃ groups at the coordinating phosphorus atom, and observed increase of the catalytic activity and shift of the products to lower molecular weight oligomer [17]. Phenacyl(diaryl)phosphine, a neutral P,O-chelating ligand, forms cationic Ni complex 1b, which catalyzes ethylene dimerization to 1-butene (TOF: 1.3 × 10⁶mol (mol cat.)⁻¹ h⁻¹) [18]. Zwitter ionic Ni complex with phosphine carboxylate ligand bonded to B(C₆F₅)₃ (1c) also functions as the catalyst for ethylene oligomerization [19].

Chart 5.1

Representative Ni and Pd catalysts for ethylene oligomerization

Diimine Ni and Pd catalysts with bulky N-aryl groups catalyze ethylene polymerization (Chap. 4). The ortho-substituents of the aryl groups prevent chain transfer reaction and produce high mass polymer. In contrast, the complexes with less hindered N-aryl groups tend to afford ethylene oligomers. Ni-diimine complexes having N-aryl groups without ortho-substituents (Chart 5.1 1d, 1e) afford linear α-olefins rather than polyethylene in the presence of MAO cocatalyst [20, 21]. Molecular weight distribution of the products obeys Schulz–Flory rule (α = 0.59 − 0.81). TOF reaches up to 136 × 10³ mol (mol cat.)⁻¹ h⁻¹ and the selectivity for α-olefin is high (up to 96 %) (Table 5.1, runs 1–5). Pd and Ni complexes with bipyrazolyl ligand [22, 23] as well as unsymmetrical bidentate N,N-ligands such as pyridylimine ligand (1f) [24] and pyridine- and imidazole-phosphinimine ligands (1g) [25] catalyze oligomerization of ethylene to afford its dimer and/or trimer. In addition to the N,N-ligands, Pd and/or Ni complexes with P,N-ligands (1h–1j) [26–31] are used for the dimerization and oligomerization of ethylene. Brookhart prepared the Pd complex with P,N-chelating ligand with an eight-membered chelate ring (1k), and found its C–H… Pd agnostic interaction in the molecular structure and catalytic activity for ethylene oligomerization [32].

Table 5.1 Ethylene oligomerization to olefins with Schulz–Flory distribution

Braunstein reported his comprehensive studies on the ethylene oligomerization catalyzed by the Ni complexes with the P–N ligands [3, 33–38]. Phosphinopyridine or phosphinooxazoline ligands show high activity for oligomerization of ethylene (3,300–29,100 g (g Ni)⁻¹ h⁻¹, TOF = 7,000–63,600 mol (mol Ni)⁻¹ h⁻¹) [3, 32–38]. Both the mononuclear and dinuclear Ni(II) complexes with the phosphinooxazoline ligands catalyze ethylene oligomerization with TOF of 36,300–61,000 mol (mol Ni)⁻¹ h⁻¹) [35–38]. The phosphinopyridine ligand with bulky substituent of the pyridine ring (ligand of complex 1i) gives the mononuclear Ni complex, which show high catalytic activity (TOF = 61,000 mol (mol Ni)⁻¹ h⁻¹) [34]. The major products are butenes (56–100 %) and hexenes (up to 41 %), and the selectivity for 1-butene is rather low (up to 39 %). Formation of the byproducts other than α-olefins are explained by (1) β-H elimination after ethylene insertion, followed by reinsertion with the opposite regiochemistry and by (2) a re-uptake mechanism for isomerization of 1- to 2-butene. Although most of the Pd and Ni catalysts for the oligomerization have divalent metal center, Hor and Braunstein reported Ni(0) complex 1 l effective for ethylene oligomerization [39].

Ni and Pd catalysts with tridentate ligands were also studied. Sun designed N,N,N-(1m and 2-imino-1,10-phenanthroline) [40–42], N,N,O-(1n) [43], P,N,N-(1o) [44], and P,N,P-(1p) [44] ligands for the Ni catalyst of ethylene oligomerization, while Liu reported structure of a cationic Pd catalyst having P,N,O-tridentate ligand (1q) [45]. The Ni complexes are activated by Et₂AlCl, Et₃Al₂Cl₃, and MAO, and catalyze conversion of ethylene into butene with C4 selectivity of 80–98 %. The Pd catalyst 1q provides higher α-olefins (C6–C16) as the product of ethylene oligomerization.

The bidentate N,N- and P,N-ligands form the catalyst of not only Ni and Pd but also Co and Fe to promote the ethylene oligomerization [46–48]. Fe and Co complexes with symmetrical and bulky bis(imino)pyridine ligand show high catalytic activity for ethylene polymerization to afford linear polyethylene [4]. Similar complexes having an ortho-substituent for each N-aryl group (Chart 5.2 2a) are effective for ethylene oligomerization to give the oligomer with Schulz–Flory distribution (α = 0.70 − 0.87). Activity is high as shown in Table 5.1, runs 10–17 (TOF up to 177 × 10⁶ mol (mol cat)⁻¹ h⁻¹) [49–52]. The catalyst with unsymmetrically substituted bis(imino)pyridine ligand (2b) also enhances the ethylene oligomerization [53, 54].

Chart 5.2

Fe and Co catalysts for dimerization and oligomerization of ethylene

Chart 5.2 summarizes Fe and Co complexes with unsymmetrical tridentate ligands. The complexes with varieties of N, N, N-(2c) [40, 55], (2d) [47, 56], (2e) [57], (2f) [58], (2g) [59], (2h) [60], and (2i) [61], were reported to catalyze the reaction. The N, N, S-, and N, N, P-tridentate ligands (2j) [62, 63], containing the diimine framework and P- or S-pendant, tend to bring about dimerization of ethylene. Typical results are summarized in Table 5.1, run 18–23. Especially, most Co complexes are effective for synthesis of 1-butene from ethylene. In contrast, the selectivity for 1-butene in C4 product is lower than the Ni-catalyzed reaction. The Fe catalysts often give longer oligomers with Schulz–Flory chain length distribution.

In addition to the late transition metal complexes, Cr complexes with meridional tridentate ligands promote ethylene oligomerization. Chart 5.3 summarizes the complex with the tridentate ligand, 3a [64], 3b [65], 3c [66], 3d [67–69], and 3e [70] (Table 5.1 run 24). Most of the complexes afford oligomers with Schulz–Flory distribution. Depending on the substituents of the ligand, the catalyst changes the product from the oligomer to polyethylene. Recently, Cr complex 3f with the tridentate ligand having two N-heterocyclic carbene sites was found to be very active for ethylene oligomerization (up to 40,440 g (mmol Cr)⁻¹ h⁻¹ bar⁻¹) (Table 5.1, runs 25, 26). In this case, the produced oligomer again obeys Schulz–Flory distribution (α = 0.46 − 0.80) [71–73]. Cr complex with phenoxy imine ligands having a pyridyl pendant at the imine nitrogen catalyzes ethylene oligomerization [74].

Chart 5.3

Representative Cr catalysts for ethylene oligomerization

5.2.3 Ethylene Oligomerization to α-Olefins with Poisson Distribution

Oligomerization of ethylene giving the product with Poisson distribution was also reported. Gibson found that ethylene oligomerization catalyzed by bis(imino)pyridine iron complex in combination with MAO and ZnEt₂ is accompanied by rapid transmetalation between alkyliron and dialkylzinc compounds. Chain growth occurs at the Fe center, and ceases after transfer of the alkyl ligand to Zn. The treatment of the resulting growing species with Ni(acac)₂ results in further transmetalation to Ni followed by β-hydrogen elimination of linear α-olefin (Scheme 5.2). The chain growth takes place in living fashion, and the produced α-olefin has Poisson distribution [75].

Scheme 5.2

Fe-Catalyzed ethylene oligomerization to α-olefins with Poisson Destribution

Hessen reported that divalent half-zirconocene alkyl complex promotes oligomerization of ethylene in the presence of 1-pentene at 20 °C [76]. Warming the reaction mixture at 50 °C leads to chain transfer of the growing species to 1-pentene to give α-olefins (both even and odd carbon numbers) with Poisson distribution.

Many other Ti and Zr complexes also show high catalytic activity for ethylene oligomerization, but they have tendency to form polyethylene and branched oligomers.

5.3 Selective Trimerization of Ethylene

As mentioned above, most of the transition metal catalysts promote the ethylene oligomerization via Cossee mechanism, to yield the products with Schulz–Flory chain length distribution. In contrast, Cr catalysts with some specific ligands have been found to promote selective trimerization and tetramerization of ethylene [2, 6, 8, 10, 11], while Ti and Ta catalysts are active for the trimerization are also known [2, 12]. Those reactions proceed via a metallacycle mechanism.

5.3.1 Chromium Catalyst

In 1967, researchers of Union Carbide Corporation found formation of 1-hexene as a predominant product during their studies of ethylene polymerization catalyzed by Cr(III)-tris-2-ethylhexanoate in combination with hydrolyzed ⁱBu₃Al cocatalyst [2]. Later, addition of dimethoxyethane to the catalyst system was found to improve the selectivity to 74 % [77, 78]. Pyrrolyl-Cr complexes also catalyze the ethylene trimerization. A number of modified Cr catalysts for ethylene trimerization were registered in their patents.

Selective formation of 1-hexene suggested a reaction mechanism distinct from the Ni, Fe, and Co catalysis, which involves Cossee-type insertion of ethylene into a metal-alkyl bond and β-hydrogen elimination and produces an oligomer mixture with a statistical distribution of the molecular lengths. Scheme 5.3 shows the proposed mechanism for the ethylene trimerization.

Scheme 5.3

Proposed mechanism for the ethylene trimerization

Oxidative coupling of ethylene on the Cr center forms chromacyclopentane. Coordination of a new ethylene molecule, and its insertion into a Cr–C bond results in formation of chromacycloheptane. A seven-membered cyclic product is kinetically less stable than the chromocyclopentane and releases the product with regeneration of low valent Cr species with π-coordinated ethylene molecules. Although these reactions would rationalize formation of 1-hexene, there may have multiple possibilities for release of 1-hexene from the metallacycle intermediate. One involves β-hydrogen elimination, forming 5-hexenyl chromium complex and reductive elimination of 1-hexene (Scheme 5.3i). A non-classic mechanism, involving 3,7-hydrogen shift of the chromacycloheptane, is also proposed for elimination of the olefin product (Scheme 5.3ii). β-Hydrogen transfer from the metallacycle to coordinated monomer would lead to reductive elimination of 1-hexene (Scheme 5.3iii), although this pathway is not discussed in most of the reports. A chromacycloheptane has been synthesized by the reaction of Cr(III) complex with 1,6-hexadienyl dimagnesium chloride, and it decomposes instantly to release 1-hexene [79].

Theoretical studies on ethylene trimerization by Cr-pyrrolyl complex and bare Cr complex have been reported [80]. DFT calculation on the Cr-pyrrolyl complexes concluded that metallacycle pathway is energetically favored and involves ring expansion as the rate-determining step. The pyrrole ligand changes its bonding mode between η⁵- and σ-ones during the reactions. Thus, the ring slippage of the pyrrole renders the reaction smooth. Cossee and the metallacycle mechanisms on chlorinated Cr-based catalysts are compared by DFT calculations [81]. The latter pathway with cationic Cr(II)–Cr(IV) intermediates is the most favored, where the rate-determining step resides in the oxidative coupling of two coordinated ethylene to form the chromacyclopentane.

5.3.1.1 Chromium Catalyst with PNP Ligands

A mixture of CrCl₃(thf)₃ and bis(diarylphosphino)amine, RN{P(C₆H₄–o–OMe)₂}₂ (Chart 5.4 4a), catalyzes trimerization of ethylene in the presence of MAO. Activity over 1 million g (g Cr)⁻¹ h⁻¹ is observed (20 bar ethylene, Table 5.2, runs 1, 2) [82]. The reaction system is thermally robust enough to be active even at 110 °C. The selectivity of 1-hexene is typically over 85 %. The ortho-OMe group may act as pendant donors, occupy the coordinatively vacant site of the chromium center, and stabilize the catalyst. The ligands with o-OEt and p-OMe substituents of the P-aryl groups and dppm backbone instead of 4a do not form the active catalyst.

Chart 5.4

Representative PNP ligands for Cr catalysts of ethylene trimerization

Table 5.2 Trimerization of ethylene

A mixture of CrCl₃(thf)₃ or Cr(acac)₃ and bis(diarylphosphino)amine ligands with 2-alkylphenyl groups at the phosphorus atoms (Chart 5.4 4b) catalyzes ethylene oligomerization to form the trimers and a smaller amount of tetramers as the major products [83]. The ligands having ortho-methyl- and ethylphenyl groups on the P atoms lead to the preferred formation of C6 products (86–93 %) (1-hexene/C-6 products >99.1 %), the activity ranging from 100,840 to 324,110 g (g Cr)⁻¹ h⁻¹ (Table 5.2, runs 3–6). Decreasing the number of ortho-substituents on the aryl groups lowers 1-hexene selectivity and enhances 1-octene formation.

Use of bis(diphenylphosphino)amine without ortho substituents forms a mixture of C6 (41.5 % yield) and C8 (41.9 % yield). The catalyst having the ligand with two ortho-methyl substituents, both symmetrical and unsymmetrical ones, results in increased formation of 1-octene. In contrast, changing the N-methyl group with N-isopropyl group leads to increased formation of C6 product.

Agapie and Bercaw synthesized and isolated triphenyl Cr complex and biphenylene Cr complex with the bis(diarylphosphino)amine ligand (Chart 5.5 5a, 5b) [84–87]. X-ray crystallography of the latter complex suggested P–O chelating coordination of a P-aryl group. The triphenyl Cr complex 5a reacts with ethylene to yield styrene and ethylbenzene rather than 1-hexene. Activation of it with H(Et₂O)₂BARF (BARF = B{C₆H₃(CF₃)₂-3,5}₄), however, provides the catalyst for 1-hexene formation with a similar activity to a mixture of CrCl₃-4a [82]. Cr complex 5b also promotes the trimerization in the presence of NaBARF. Thus, the catalytic trimerization catalyzed by a mixture of Cr salt, 4a, and MAO involves a cationic Cr complex as the active species. Trimerization of an equimolar mixture of C₂H₄ and C₂D₄ catalyzed by 5a/H(Et₂O)₂BARF or 5b/NaBARF affords only four isotopomers C₆D₁₂, C₆D₈H₄, C₆D₄H₈, and C₆H₁₂ in a 1:3:3:1 ratio (Scheme 5.4i). The reaction proceeds via a metallacycle route rather than via repetition of Cossee-type insertion and β-hydrogen elimination, because isotopomers containing an odd number of deuterium due to H/D scrambling are not found in the reaction mixture. The reaction of cis-1,2-dideuterioethylene by 5b/NaBARF gives a mixture of two isotopomers with CHD=CH- and CDH=CD- fragments in 1:2.4 ratio and no H/D scrambled isotopomers having a CH₂= or CD₂= group (Scheme 5.4ii). The 1,1-dideuterioethylene also produces four isotopomers shown in Scheme 5.4iii, selectively.

Chart 5.5

Cr complexes used as the catalysts of ethylene trimerization

Scheme 5.4

Trimerization of deuterated ethylenes by Cr catalysts with bis(diarylphosphino)amine ligand

All these results suggest intermediacy of the chromacycloheptane in the ethylene trimerization. Kinetic isotope effects of the reaction indicated that the rate-determining step of the reaction should involve C–H bond cleavage process such as β-hydrogen elimination from the chromacycloheptane, giving a hydride(5-hexenyl)chromium species, or 3,7-hydrogen shift of the metallacyclic intermediate.

Trichlorochromium(III) complex having ligand 4a (Chart 5.5 5c) shows dynamic NMR spectra due to fluxional behavior of the ether groups interchanging on the NMR time scale [86]. Oxidation of Cr(0)-carbonyl complex 5d (Chart 5.5) with [AcFc]BF₄ yields cationic Cr(I) complex, [Cr(CO)₄(4a)]BF₄, which does not catalyze ethylene trimerization [88]. Addition of AlEt₃ to the cationic complex causes elimination of a CO ligand and starts to catalyze ethylene trimerization. The ligand with 2-(methylthio)phenyl groups prefers a SPS coordination rather than the OPP coordination.

Cr(0) complex with a PNPNH ligand (4c), 5e (Chart 5.5), catalyzes ethylene trimerization in the presence of Et₃Al (activity = 289 g (g Cr)⁻¹ h⁻¹, C6 = 86 %, 1–C6 = 98 %) (Table 5.2, run 7) [89–91]. The reaction obeys first-order kinetics to ethylene and catalyst concentrations with the activation energy of 52.6 kJ mol⁻¹ [90]. A mixture of CrCl₃(thf)₃ and the aluminum adduct of PNPNH (Chart 5.4 4c) or aluminum amide, formed by the reaction of 4c with organoaluminum, is also active for the trimerization. Cr(acac)₃/4c/AlEt₃, on the other hand, does not promote the reaction, whereas addition of Et₄PCl makes it active for the selective ethylene trimerization (activity = 26,700 g (g Cr)⁻¹ h⁻¹, C6 = 93.0 %, 1–C6 = 99.0 %) (Table 5.2, run 8). Al/Cl ratios in the reaction using CrCl₃ as the catalyst precursor affect the catalytic activity, indicating the importance of chloride in activation of the Cr–Cl bonds.

Cr complex with a bidentate PCNCP ligand (Chart 5.5 5f) promotes ethylene trimerization upon activation with MAO (activity up to 9,783 g (g Cr)⁻¹ h⁻¹, C6 = 99 %, 1–C6 = 98 %) (Table 5.2, run 9, 10) [92]. The ligand is coordinated by the Cr center in a P,P-bidentate form. The reaction using the ligand with sterically bulky substituents on the P and N atoms keeps selectivity of 1-hexene, while use of less bulky substituents causes increased formation of C8 products in addition to the C6 products. DFT calculations of the intermediate indicate that formation of Cr(I)-(1-hexene) complex from chromacycloheptane complex proceeds via intramolecular 3,7-hydrogen shift and not from β-hydrogen elimination/reductive elimination.

5.3.1.2 Chromium Catalyst with Tridentate PNP or SNS Ligands

Tridentate ligands listed in Chart 5.6 are used for the ligands of Cr catalyst for ethylene trimerization. Cr complex with P,N,P-ligand 6a (R = Et) shows high activity and excellent selectivity for the ethylene trimerization [93]. TOF of the catalyst attains to 69,340 mol (mol Cr)⁻¹ h⁻¹ with 99.1 % selectivity for 1-hexene (Table 5.2, runs 11, 12).

Chart 5.6

Representative tridentate ligands for Cr-catalyzed ethylene trimerization

An S,N,S-ligand 6b (Chart 5.6) reacts with CrCl₃ to form trivalent complex, CrCl₃(6b), having a meridional tridentate ligand bonded to an octahedral Cr center. The structure is similar to the tridentate PNP complex, and the chelating bite angels of the ligands, Cr–S or Cr–P distances, and Cr–N distances are almost similar between the SNS and PNP complexes. Complex of 6b (R = Et) shows high activity (up to 160,840 g (g Cr)⁻¹ h⁻¹) and selectivity (98.4 % selectivity for 1-hexene and 99.7 % selectivity for C6 product) for ethylene trimerization (Table 5.2, runs 13) [94]. Cr(III) catalyst having 6b (R = decyl) improves the catalytic activity on addition of MAO (30–100 eq to Cr), partly due to enhanced solubility of the ligand in the solvent.

Gambarotta and Duchateau chose 6b and 6c, having a pyridyl group as the coordinating group, as the supporting ligand of the Cr complexes and investigated details of their activation by cocatalysts. Scheme 5.5 summarizes the reaction of organoaluminum compounds with the Cr complexes having the SNS ligands.

Scheme 5.5

Reaction of Cr complex having SNS ligand with aluminum compounds

AlCl₃ converts CrCl₃(6b) (R = cyclohexyl) to mono- and di-nuclear cationic Cr complexes A and B, depending on the Al to Cr ratio [95]. Addition of excess Me₃Al (10 equiv) to a mixture of CrCl₃ and 6b causes formation of a cationic dinuclear Cr complex {(6b)CrMe(μ-Cl)}₂{(Me₃Al)₂(μ-Cl)}₂ (C). A similar cationic complex with ethyl ligands, {(6b)CrEt(μ-Cl)}₂-[EtAlCl₃]₂, was obtained by the reaction of CrCl₃(6b) with AlEt₂Cl [96]. The complex does not undergo β-hydrogen elimination from the Cr-Et group even in the presence of excess cocatalyst. By the addition of MAO, these complexes catalyze ethylene trimerization with a similar reactivity to CrCl₃(6b). The reaction of CrCl₃(6b) with isobutylaluminoxane (iBAO) leads to dinuclear divalent Cr complex (Scheme 5.5 D), which also acts as the selective trimerization catalyst. Thus, the trivalent Cr complex is general precursors, and yields a catalytically active divalent species in the presence of the cocatalyst. Addition of excess alkylating agents (MAO, Me₃Al, Et₃Al), however, degrade the Cr(III) complexes rapidly.

Cr(III) complexes with 2,6-bis(thiolatomethyl)pyridine (6c, R = Ph, Cy) also catalyze ethylene trimerization [97]. Reaction of 6c with CrCl₂(thf) ₂ causes formation of the Cr(II) complex. Although the Cr(III)-6c complex converts ethylene to 1-hexene selectively in moderate activity, CrCl₂(6c) promotes the oligomerization of ethylene to the product with a statistical distribution of molecular weights.

Further studies on the tridentate ligands with various substituents, donor atoms, and structures were conducted [98, 99]. Introduction of Me or benzyl substituents at N atom of the PNP ligand leads to dramatic decrease of the productivity and selectivity for 1-hexene, accompanied by increase of the polymer. The complex with the SNS ligand having a trimethylene spacer, 6d, also catalyzes selective trimerization, but the activity and selectivity for C6 product is lower than the ligand with ethylene spacer only (activity = 14,770  (g Cr)⁻¹ h⁻¹, C6 = 81 %, 1–C6 = 97.9 %, Table 5.2, run 14). Cr complex with Ph₂PCH₂CH₂SCH₂CH₂PPh₂ (tridentate PSP) ligand does not cause selective trimerization, but produces a mixture of the oligomers with Schulz–Flory distribution. Similar PSP Cr complex with Et-P group results in predominant formation of 1-hexene, although the activity and selectivity are lower than those with PNP and SNS ligands also [99]. Cr complex with EtSCH₂CH₂PPhCH₂CH₂SEt (tridentate SPS) ligand shows almost similar result to that with the PSP ligand with PEt₂ groups. McGuinness also prepared Cr(II) and Cr(III) complexes with tridentate PNP or SNS ligands. The performance of Cr(II) precatalysts is comparable with their Cr(III) counterparts on MAO activation. These ligands are easily deprotonated by added base to yield the active complexes for ethylene trimerization.

Bluhm reported the Cr complexes with PNP and SNS ligands having ortho-phenylene spacer between the P or S atom and N atom, and/or imine structure (Chart 5.6 6e) [100]. Meridional tridentate coordination of these ligands is confirmed by X-ray crystallography. The complex with PNP and PNS type ligands with imine center brings about selective formation of 1-hexene (C6 = 82–98 %) with good activity (TOF = 2,294–5,742 mol (mol Cr)⁻¹ h⁻¹) (Table 5.2, run 15, 16), but other complexes tend to afford polyethylene.

5.3.1.3 Chromium Catalyst with Facial Tridentate Ligands

Braunstein and Hor reported that Cr(III) complexes with heteroscorpionate pyrazolyl ligands are effective for selective trimerization of ethylene [101] (Chart 5.7). The C6 selectivity of tris(pyrazolyl)methane Cr complex (Chart 5.7 7a) is 97.6 wt%, and the activity is up to 32,400 g (g Cr)⁻¹ h⁻¹ (Table 5.2, run 17) [102]. The facial tridentate coordination of the ligand is conformed by X-ray crystallography.

Chart 5.7

Representative Cr complexes with heteroscorpionate ligands for ethylene trimerization

Cr complex with bis(pyrazolyl)(imidazolyl)methane ligand (Chart 5.7 7b) shows improved activity (53,000 g (g Cr)⁻¹ h⁻¹) retaining high selectivity (C6 = 98.5, 1–C6 = 99.1 %) (Table 5.2, run 18). Complexes 7c and 7d also catalyze the reaction with similar selectivity (Table 5.2, run 19, 20) [102]. Further crystallographic analysis on the intermediate formed by the reaction of the Cr complex and organoaluminum [103]. Tris(pyrazolyl)methane Cr complex (Chart 5.7 7a) reacted with Me₃Al (6 equiv.) and with MAO (10 equiv.) to produce the corresponding cationic dinuclear Cr(II) complex and Me₂AlX adduct of Cr(III)Cl₂Me complex (Scheme 5.6i).

Scheme 5.6

Reaction of Cr complexes with heteroscorpionate ligands with organoaluminums. X = Cl or Me

Further addition of 2 equiv. of Me₃Al converts the Cr(III)Cl₂Me complex to the cationic Cr(II) complex. The reaction with Me₃Al-free MAO affords neither of the complexes, indicating Me₃Al is active reductant. As the cationic Cr(II) complex shows very low activity for ethylene trimerization in the presence of Me₃Al, MAO is also essential for the trimerization process. The reaction of excess Me₃Al with the bis(pyrazolyl)methane benzylamine Cr complex (Chart 5.7 7d (E = NH)) results in dehydrogenation from the NH and aryl CH groups of the ligand to give a heterobimetallic Cr–Al complex (Scheme 5.6ii). Thus, Me₃Al plays varied roles in the catalysis.

Duchateau and Mountford also studied Cr catalysts with various heteroscorpionate ligand, including bis(pyrazolyl)methane with pendant secondary and tertiary amine donors as well as phenol and phenyl ether donors [104]. Most of the complexes show improved activity compared to the complex with tris(pyrazolyl)methane ligand (up to 3,250 g (mmol Cr)⁻¹ h⁻¹).

5.3.1.4 Single Component Chromium Catalyst

Cr complexes with the phosphenimidous diamide ligands act as the single component catalyst for the ethylene trimerization [105, 106]. Reaction of ⁱBu₃Al with [(^tBu₂N)₂P]₂Cr affords complex E or F depending on the Al/Cr ratio (Scheme 5.7).

Scheme 5.7

Single component Cr catalysts formed from Cr complexes having (R₂N)₂P or (^tBu)N-PPh₂ ligands

The complexes catalyzed ethylene polymerization while addition of ⁱBu₃Al cocatalyst to the reaction mixture changes the product to 1-hexene. Al–Cr heterobimetallic complex (G) obtained from the reaction of Me₃Al with [(^tBu₂N)₂P]₂Cr (4:1) (Scheme 5.7i) catalyzes the trimerization of ethylene without the cocatalyst (600 g (mmol Cr)⁻¹ h⁻¹, C6 = 99.9 %) [105].

An equimolar reaction of vinylmagnesium chloride with [NPN]CrCl₂Li(thf)₂ affords trinuclear Cr complex (H) (Scheme 5.7ii) [107]. Despite apparent Cr(II)/Cr(I) mixed valence species, DFT calculations revealed that all of the Cr atoms are divalent. The complex promotes ethylene trimerization without cocatalyst (1,740 g (g Cr)⁻¹ h⁻¹).

Dinuclear mixed-valence Cr(I)/Cr(II) complex with Ph₂P-N(^tBu) ligand, J, is obtained by the reaction of tetranuclear Cr complex I with PMe₃ and KC₈, and promotes oligomerization of ethylene to give a mixture of 1-butene and 1-hexene (Scheme 5.7iii) [108]. Activation of the complex with DMAO/Et₃Al cocatalyst, causes selective trimerization. In contrast, the activation with Et₃Al and with DMAO (dried MAO) results in selective dimerization and polymerization, respectively.

Dinuclear Cr complex, obtained by the reaction of CrCl₂(thf)₂, Me₃Al, and tetramethylpyrrole (Eq. 5.1), is also active for ethylene trimerization in methylcyclohexane in the absence of cocatalyst (activity = 670,000 g (mol Cr)⁻¹ h⁻¹, C6 = 95 %) [109].

(5.1)

Use of toluene as the solvent decreases selectivity for C6 fraction (C4 = 46 %, C6 = 39 %). The Cr complex in combination with cocatalyst does not change the product selectivity significantly. The complexes obtained from CrCl₃(thf)₃ catalyze ethylene polymerization without a cocatalyst.

5.3.2 Titanium Catalyst

In 2001, Hessen reported trimerization of ethylene catalyzed by CpCMe₂PhTiCl₃/MAO (Chart 5.8 8a) with catalytic activity of 6,292 kg (mol Ti)⁻¹ h⁻¹ (Table 5.2, run 21, 22) [110]. The reaction proceeds at 30 °C and the selectivity for 1-hexene is 83–87 wt%. C10 products (mainly 5-methylnon-1-ene, >75 %) are also formed as a result of cotrimerization of ethylene and 1-hexene (9–14 wt%). A similar Ti complex without the aryl group of the ligand affords polyethylene rather than 1-hexene.

Chart 5.8

Representative Ti catalyst for ethylene trimerization

Accompanying formation of high molecular weight polyethylene lowers yield of 1-hexene and causes reactor fouling. Detailed investigation of the reaction revealed that polyethylene is formed at early stage of the reaction, which is promoted by partly alkylated titanium species. The amount of produced polyethylene can be largely reduced if the complex is premixed with MAO prior to injection into the reaction mixture. Type of organoaluminum cocatalyst is also important for the reduction of polyethylene. The use of MAOs that do not contain and/or are not able to generate aluminum hydride species increases productivity of 1-hexene and depress the polyethylene formation.

Pendant aryl groups and bridging groups between the Cp and aryl groups of the ligand influence selectivity of 1-hexene [111]. The highest activity and selectivity for trimerization is obtained for the catalysts with isopropylidene-bridged 3,5-dimethylphenyl group (Table 5.2, run 23). A SiMe₃ substituent on the Cp ligand improves the catalyst activity and selectivity, whereas methyl substituent on the aryl group decreases activity.

Coordination of the pendant arene moiety to the titanium center is confirmed for [CpCMe₂C₆H₃Me₂-3,5]TiMe ⁺₂ , [CpCMe₂CH₂C₆H₃Me₂-3,5]TiMe ⁺₂ , and [Me₃SiCpCMe₂C₆H₃Me₂-3,5]TiMe ⁺₂ in the solid state [112]. NMR analysis of [Me₃SiCpCMe₂C₆H₃Me₂-3,5]TiMe ⁺₂ showed exchange of coordinated faces of the aryl group on the NMR time scale, suggesting labile nature of the aryl pendant group.

Huang also conducted ethylene trimerization by using half-titanocene having pendant thienyl group [113]. Although the complex with 1-thienyl group (Chart 5.8 8c) catalyzes selective formation of 1-hexene (84 wt% at 30 °C, 156 kg (mol Ti)⁻¹ h⁻¹, Table 5.2, run 24), but catalyst with 2-thienyl group forms 1-hexene in 12 wt%, accompanied by the formation of polyethylene. Similar half-titanocene complexes with ether pendant also promote ethylene trimerization, but the activity is lower than those with thienyl group (25–57 kg (mol Ti)⁻¹ h⁻¹) [114].

The Ti-catalyzed trimerization of ethylene using the complex with a Cp ligand having aryl pendants also proceeds via a metallacycle mechanism (Scheme 5.8).

Scheme 5.8

Proposed mechanism for the Ti-catalyzed ethylene trimerization

Neutral trichlorotitanium(IV) complex is converted into the cationic Ti(II) species with an ethylene ligand (Scheme 5.8i) which initiates the catalytic ethylene trimerization [115]. The aryl pendant group assists to make the above reaction smooth by decoordination and recoordination.

DFT calculation of the ethylene trimerization by a cationic (C₆H₅CH₂C₅H₄)Ti fragment supported the mechanism involving metallacycloheptane, formed by oxidative cyclization of Ti complex with ethylene followed by ethylene insertion [116]. Formation of 1-hexene from the titanacycloheptane intermediate occurs via direct β-hydrogen shift rather than via the β-hydrogen elimination/reductive elimination. The β-hydrogen shift takes place through a transition state with a nearly-linear C–H–C arrangement. Formation of 1-butene from a titanacyclopentane intermediate takes place via the other two-step pathway (β-hydrogen elimination/reductive elimination). High energy barrier for this pathway (41 kcal mol⁻¹) renders the formation of 1-butene difficult. The reaction of ethylene with titanacycloheptane to give titanacyclononane is also disfavored compared to the elimination of 1-hexene from titanacycloheptane. Thus, 1-hexene is produced predominantly from the reaction. The pendant arene moiety is more strongly bound to the Ti(II) species rather than the Ti(IV) species of the reaction. The role of the pendant arene is to reduce the olefin coordination energy and thus to promote 3,7-hydrogen shift over further growth of the metallacycle.

DFT (B3LYP functional) studies of the Ti-catalyzed ethylene trimerization concluded the mechanism involving β-hydrogen shift rather than β-hydrogen elimination and reductive elimination [117]. They also suggested easier β-hydrogen shift from the titanacycloheptane and high barrier for the formation of 1-butene from titanacyclopentane complex, due to the geometrical constraints in opening the five-membered metallacycle. The rate-determining step is the ring-opening reaction of the seven-membered metallacycle (barrier is 18.4 kcal/mol). Replacement of the aryl pendant group of the ligand with a non-coordinating methyl group changes the favorable product to polyethylene, which is in agreement with the experimental results.

Tobisch investigated on comparison of the possibilities for the titana(IV)cycle intermediates to cause growth or to decompose affording α-olefins as a function of their ring size, prediction of the favorable route for precatalyst activation, and exploration of the cycloalkane production as a possible side process, by using a gradient-corrected DFT method [118–121]. Metallacycle growth through bimolecular ethylene uptake and subsequent insertion displays very similar structural and energetic characteristics for five- and seven-membered titana(IV)cycles. Decomposition of titana(IV)cycles to α-olefins preferably takes place via a concerted transition-metal-assisted β-hydrogen shift for conformationally flexible metallacycles, with the barriers having to be overcome. The rigid five-membered titana(IV)cyclopentane, however, does not undergo the β-hydrogen shift due to the kinetic barrier and chooses further ethylene insertion, forming the titana(IV)cyclopentane with a seven-membered ring. On the basis of the detailed insights into the ability of titana(IV)cycles to undergo either growth or decomposition to α-olefins, the thermodynamic and kinetic aspects for the selectivity control of the linear ethylene oligomerization have been rationalized.

Quite recently, Kawamura and Fujita reported that Ti complex with phenoxy-imine ligand with pendant aryl-OMe pendant 8a (Chart 5.8) is effective for trimerization of ethylene to give 1-hexene with very high productivity (6,590 kg (g Ti)⁻¹ h⁻¹) (Table 5.2, run 25) [12, 122]. Selectivity for C6 product is 92.3 wt%. Similar complex with aryl-OPh pendant shows lower activity to form a considerable amount of polyethylene. DFT calculations suggest that Ti–OR bond distance is shorter in the complex with aryl–OMe group than aryl–OPh group. Ethylene pressure studies indicate second-order dependence of productivity on ethylene pressure, which support the metallacyclic mechanism. The byproducts other than polyethylene contain dodecene (2-butyl-1-hexene, mainly (ca. 90 wt%)), formed by cotrimerization of 1-hexene with ethylene.

5.3.3 Tantalum Catalyst

In 2001, Sen reported that TaCl₅ in combination with alkylating agents promotes selective trimerization of ethylene in the absence of a ligand [123] (Chart 5.9 9a). The reaction proceeds at 45–60 °C under 700 psi of ethylene to produce a mixture of 1-butene, 1-hexene, and 1-octene, where the selectivity for 1-hexene is >94 % (TOF in 385–460 mol (mol Ta)⁻¹ h⁻¹, Table 5.2, run 26, 27). Alkylating reagents, Me₄Sn, Me₂Zn, Me₃Al, and alkyl lithiums are effective as the additive. Higher alkylating reagents and alkyl lithium are less useful.

Chart 5.9

Representative Ta catalyst for ethylene trimerization

The mechanism of generation of the active species in the catalysis is proposed as shown in Scheme 5.9. It involves reduction of Ta(V) to Ta(III) in the reaction of TaCl₅ with the alkylating agents, formation of tantalum(V) metallacyclopentane by the reaction of Ta(III) with two molecules of ethylene. Insertion of another molecule of ethylene to its carbon–tantalum bond, β-hydrogen elimination to afford Ta(V) alkylhydride, and reductive elimination to afford 1-hexene with regeneration of Ta(III).

Scheme 5.9

Formation of tantalacyclopentane complex in the reaction of TaCl₃Me₂ and ethylene

MP2 and B3LYP calculations clarified details of the ligand-free reaction [124]. As shown in Scheme 5.9, dimethyltantalum complex, formed by the metathesis of TaCl₅ with methylating agents, adopts trigonal bipyramids with two Cl ligands at the axial positions. It allows insertion of ethylene to its methyl-tantalum bond via Cossee mechanism, followed by β-hydrogen elimination to give Ta(III) species with liberation of methane. After formation of the Ta(III) species, the catalytic cycle starts, similarly to those shown in Cr- and Ti-catalyzed selective trimerization of ethylene. Tantalacyclopentane complex is transformed to tantalacycloheptane complex by the insertion of ethylene (E _a = 13.0 kcal mol⁻¹). Further ethylene insertion of tantalacycloheptane to tantalacyclononane requires energy barriers of 36.3 kcal mol⁻¹, which is much larger than that of the transformation of tantalacycloheptane to TaCl₃(1-hexene) accompanied by elimination of the product (25.5 kcal mol⁻¹). Thus, trimerization is favored rather than tetramerization and dimerization.

Recently, Mashima reported that catalysts composed of TaCl₅ in combination with bis(trimethylsilyl)cyclohexadiene (Chart 5.9 9b) or its derivatives promote selective trimerization of ethylene to give 1-hexene [125]. The selectivity for 1-hexene is up to 98.5 % and TOF is up to 1,008 mol (mol Ta)⁻¹ h⁻¹ (Table 5.2, run 28, 29). Similar to the above catalytic system, Ta(III) species is formed by the reaction of TaCl₅ and bis(trimethylsilyl)cyclohexadiene, which is active species of the reaction. In situ NMR analysis of the mixture of TaCl₅ and bis(trimethylsilyl)cyclohexadiene in the presence of ethylene at −10 °C shows the formation of tantalacyclopentane. Upon warming the reaction mixture to room temperature, the signals due to the metallacycle disappear and those due to 1-hexene become observable. The formation of Ta(III) species is also supported by the isolation of the corresponding alkyne complex.

5.3.4 Ruthenium Catalyst

Transition metal catalysts other than Cr, Ti, and Ta, which are active for selective trimerization of ethylene, are very limited. Recently, however, Kondo reported Ru(0) complex brings about selective trimerization of ethylene (Scheme 5.10) [126]. The product is a mixture of isohexenes (94 %) and 2-hexene (6 %). The catalyst also promotes codimerization of ethylene with 1-butene or (E)-2-butene to give isohexene.

Scheme 5.10

Trimerization of ethylene by ruthenium catalyst

5.4 Tetramerization of Ethylene

Cr complexes with the PNP ligands are one of the most effective catalysts for trimerization of ethylene, giving 1-hexene selectively. As mentioned above, the trimerization proceeds via a metallacycloheptane intermediate, which undergoes β-hydrogen shift or β-hydrogen elimination/reductive elimination. Although insertion of another ethylene molecule to the metallacycloheptane would lead to 1-octene, via metallacyclononane, such selective tetramerization is much rarer than the trimerization. A nine-membered ring is the most unfavored medium-sized ring in organic chemistry, and formation of the metallacyclononane was considered to an unfavorable process. Recently, Cr complexes with special PNP ligands have been found to catalyze tetramerization of ethylene effectively.

5.4.1 Effect of Ligand Structure

The catalyst prepared from CrCl₃(thf)₃ bis(diphenylphosphino)amine (PNP), and MAO (Chart 5.10 10a) causes ethylene oligomerization to give 1-octene and less amount of 1-hexene [127, 128]. The isolated Cr catalyst adopts dimeric structure [(10a)CrCl₂]₂(μ-Cl)₂ and shows activity for tetramerization in the presence of MAO [127]. The PNP compounds with 3- and 4-methoxyphenyl groups at the P atom also function as the catalyst ligand for ethylene tetramerization. The ligands with varieties of N-substituents have been examined for the catalyst (Table 5.3, runs 1–12) [129–131]. The catalysts with an N–H group afford a mixture of α-olefins with broad distribution, without specific selectivity for C6 and C8, probably due to the deprotonation of NH group. On the other hand, introduction of methyl group on N atom increases C8 selectivity to 59.0 % (94.1 % α-selectivity) and C6 selectivity to 24.8 % (39 % α-selectivity) and 55 % cyclic products (Table 5.3, run 1).

Chart 5.10

Representative ligands for Cr-catalyzed ethylene tetramerization

Table 5.3 Tetramerization of ethylene

Substituent on the N atom affects the catalyst productivity and selectivity [129]. The ligand with a longer alkyl group on the N atom improves catalytic activity, but the selectivity is not so much influenced (Table 5.3, runs 2, 3). Highest α-selectivity of C8 products is obtained by the ligand with isopropyl and cyclohexyl substituents (α-selectivity >99 %), indicating secondary alkyl substituent on N atom increases the α-selectivity (Table 5.3, runs 4, 5). Alkyl groups with β-branching such as benzyl group also increase the α-selectivity of C6 by decreasing the amount of cyclic product. Although the above reactions are conducted in toluene, the use of methylcyclohexane solvent greatly improves the catalytic activity [129].

Activity and selectivity of the Cr catalyst of bis(diphenylphosphino)amine ligand with an N-cycloalkyl substituent are affected by its ring size [130]. The catalysts with smaller size ring (cyclopropyl and cyclobutyl) at the N atom cause lower α-selectivity for both C6 and C8 products with lower productivity (Table 5.3, runs 10, 11), whereas the increased α-selectivity and higher productivity are achieved as increasing the ring size of the substituents (Table 5.3, runs 12–14). With N-cyclododecyl substituents, the α-selectivity for C6 and C8 products is increased to 84.6 and 99.4 %, respectively, and productivity reaches 757,720 g (g Cr)⁻¹ h⁻¹ (Table 5.3, run 14). The selectivity of 1-octene is highest in the reaction using the ligand with an N-cycloheptyl group (C8 = 68.1 wt%) (Table 5.3, run 13). The introduction of 2-alkyl substituents on N-cyclohexyl group increases formation of 1-hexene over 1-octene in very high productivity (>2,000,000 g (g Cr)⁻¹ h⁻¹). This result is in agreement with that the increment of steric bulk in P-substituents favors trimerization rather than tetramerization.

Bis(diphenylphosphino)amine ligands with N-aryl substituents are also effective (Table 5.3, run 15) [131]. Similar to the results of N-alkyl functionalities, introducing bulky isopropyl substituent on ortho-position of the aryl group increases C6 and 1–C6 selectivities from 16.6 and 54.2 % to 33.4 and 85.4 %, although the catalyst productivity drops from 765,900 to 159,600 g (g Cr)⁻¹ h⁻¹). The use of N-benzyl group instead of N-phenyl group does not show any remarkable change in product selectivity, but the catalyst productivity is increased to 1,065,300 g (g Cr)⁻¹ h⁻¹ (Table 5.3, runs 16). Furthermore, the ligand with N-phenethyl group improves α-selectivity (84.3 %) compared to that with N-benzyl group (70.5 %) without significant loss of productivity (1,001,600 g (g Cr)⁻¹ h⁻¹). On the whole, PNP ligands with N-aryl functionality show lower selectivity than their N-cyclohexyl analogues, both in terms of overall α-octene formation and overall α selectivity.

The complexes with methoxyalkyl or methoxyaryl group on N atom show lower activity than favor formation of C6 product (61–66 wt%) rather than C8 product (24–34 %), but the increased ethylene pressure favors 1-octene production (Table 5.3, runs 17–20) [132]. Similar complexes with thioether pendant on N atom (Chart 5.10 10d), in contrast, promote ethylene tetramerization (C8 = up to 55.5 wt%) in the presence of MAO (Table 5.3, runs 21–24) [133]. In contrast, thiophenyl group results in increased selectivity (68.3 %). Organoaluminum cocatalysts other than MAO, such as MMAO and EAO, are usable as the cocatalyst. In addition to PNP ligand, PNNP ligand (Chart 5.10 10c) is effective for tetramerization (Table 5.3, run 25).

Overett reported varieties of carbon-bridged diphosphine ligands for the ethylene oligomerization including tetramerization [134]. Although Cr-dppm complexes give ethylene oligomers with Schulz–Flory distribution (α = 0.55), the catalysts with dppe and dppp form 1-octene in 59.3 and 30.3 wt%, respectively, with lower activity (144,000 and 13,000 g (g Cr)⁻¹ h⁻¹) and increased formation of polyethylene compared to PNP (Table 5.3, runs 26–28). Use of 1,2-diphenylphosphinobenzene 10e causes the tetramerization in high productivity (Table 5.3, run 29) [134].

Cr catalysts with PNP ligand are used for both ethylene trimerization and tetramerization (Chart 5.4 and 5.10). The key for the formation of 1-octene rather than 1-hexene is the P-substituents of the ligand. The ligand with ortho-methoxyphenyl substituents on P atom causes selective trimerization, and that with para-methoxyphenyl substituents enhances tetramerization rather than trimerization. It may be due to the steric crowding around the catalytic center or by pendant coordination of a donor substituent.

P–Cr–P bite angle is related to the oligomerization results. The bis(diarylphosphino)amine ligand, which forces small bite angles (ca. 67°), shows the highest 1-octene:1-hexene ratios (ca. 9.1:1), although the ratio is largely influenced by the N-substituent. The 2-carbon spacer ligands, such as dppe are typically coordinated by transition metals with the P–M–P bite angles of 81–83°, and produce slightly more 1-hexene at the expense of 1-octene. The 3-carbon spacer ligands with still larger bite angles—typically ca. 91° for dppp, gave the lowest 1-octene:1-hexene ratios. Bis(diphenylphosphino)benzene show very high catalyst activity in those carbon spacer ligands, but the selectivity is inferior compared to the bis(diarylphosphino)amine ligand due to increased formation of C6 cyclic products and low 1-octene to 1-hexene ratio. Similar 1,2-phenylene bridged ligand with P-(2-ethylphenyl) group or P-isopropyl group leads to the increased selectivity for trimerization of ethylene (1-hexene = 59.2 and 82.8 %, respectively). The use of bis(diisopropylphosphino)ethane and bis(dimethylphosphino)ethane ligand also shifts the reaction from tetramerization to trimerization, compared to dppe. Oligomerization using dppm lacks selectivity, but bis(diisopropylphosphino)methane, with more basic nature, is effective as ligand for selective tetramerization. Thus, both steric and electronic factors are important in the selectivity.

Cheong employed Cr complexes with stereoisomers of 1,2-dimethyl-1,2-bis(diarylphosphino)ethane ligands and obtained catalytic activity of 274–2,256 kg (g Cr)⁻¹ h⁻¹ for ethylene tetramerization [135]. The racemo-(S,S) or (R,R) complexes show higher activity and 1-octene selectivity than meso-(S,R) complex. The complex with racemo-1,2-dimethyl-1,2-bis(diphenylphosphino)ethane ligand shows catalytic activity of 1,929 kg (g Cr)⁻¹ h⁻¹), which is higher than that using PNP (282 kg (g Cr)⁻¹ h⁻¹) under the same conditions, and good selectivity for 1–C8 (59.2 %). X-ray crystallography and DFT calculation showed smaller PCCP dihedral angle and smaller P–Cr–P bite angle of racemo complex compared to the meso complex.

In line with the founding by Overett, the product obtained by Cr dppm complex is a Schulz–Flory distribution mixture. Wass reported the synthesis of cationic Cr(I) complexes with alkylated dppm ligands and their use for ethylene oligomerization [136]. The complexes with alkyl group attached to carbon bridge of the ligand afford C8 product mainly (30.8–38.4 %), although their selectivity and activity (44,080–70,150 g (g Cr)⁻¹ h⁻¹) are inferior to that with PNP ligand (Table 5.3, runs 30–32).

Hanton obtained [Cr(CO)₄(PNP)][Al{OC(CF₃)₃}₄] by the reaction of Cr(CO)₄(PNP) with Ag[Al{OC(CF₃)₃}₄], and analyzed its structure by X-ray crystallography [137]. The complex promotes tetramerization of ethylene in the presence of AlEt₃, although similar complexes with PF₆ or BF₄ counter anion are not active.

5.4.2 Effect of Cocatalysts and Additives

Various cocatalysts were examined for the ethylene trimerization/tetramerization catalyzed by the Cr complexes. AlEt₃-based cocatalysts for the reaction using a mixture of CrCl₃(thf)₃, bis(diphenylphosphino)isopropylamine as the catalyst change the selectivity from 90 % C6 to 72 % C8 depending on the cocatalysts [138, 139]. B(C₆F₅)₃ and Al(OC₆F₅)₃ are not suitable for the cocatalysts, but [Ph₃C][Al{OC(CF₃)₃}₄] affords highly active and long-lived catalysis system.

Jiang reported addition of tetrachloroethane (10 mol equiv. with respect to Cr) to the PNP/Cr/MAO system improved selectivity of 1-octene from 71.8 % (activity = 18.8 × 10⁶ g (mol cat.)⁻¹ h⁻¹) to 74.9 % (activity = 3.42–18.8 × 10⁶ g (mol cat)⁻¹ h⁻¹) [140]. Dichloromethane and 1,1,2-trichloroethane are superior with respect to 1-octene selectivity and catalytic activity, compared to trichloromethane, tetrachloromethane, and 1,1,1-trichloroethane [141]. These chlorides work better than the corresponding bromides. These alkyl chloride additives were proposed to coordinate to two chromium centers of the dinuclear complex to change this structure a more suitable one for ethylene tetramerization.

5.4.3 Mechanism of Cr-Catalyzed Ethylene Tetramerization

Scheme 5.11 shows the proposed mechanism for ethylene tetramerization. Detailed studies on the reaction products rationalized formation of 1-butene, 1-hexene, methylcyclopentane, methylenecyclopentane, 2-propenylcyclopentane, n-propylcyclopentane, and C10, C12, and C14 secondary oligomerization products [142]. The reaction of a mixture of C₂H₄ and C₂D₄ and analysis of the methylcyclopentane isotopomer distribution revealed that the reaction proceeds via metallacycle formation, not via Cossee–Arlman linear chain growth.

Scheme 5.11

Proposed mechanism for the Cr-catalyzed ethylene tetramerization

Enhanced stability of metallacycoheptane intermediate of the tetramerization relative to that of the trimerization catalysis renders insertion of ethylene, forming 1-octene, competing with formation of C6 products such as 1-hexene, methylenecyclopentane, etc.

Gambarotta isolated a new cationic complex {[PNP]₂Cr(μ-Cl)AlMe₃}[ClAlMe₃]_0.34[Me₄Al]_0.66 from the reaction mixture of PNPCrCl₃ with AlMe₃ [143]. Upon addition of MAO, it promotes trimerization and tetramerization of ethylene (C8: 73.4 %) with activity of 8,400 g (g Cr)⁻¹ h⁻¹, which is comparable to [PNP]CrCl₃/MAO catalyst (8,000 g (g Cr)⁻¹ h⁻¹). Similar activity of the Cr(III) precatalyst and the Cr(II) complex for the tetramerization suggests that Cr(III) is reduced to Cr(II) at a preliminary stage of the catalysis.

The ligand-to-Cr ratio changed the reactions from selective 1-hexene and 1-octene formation to formation of the oligomers with statistical molecular length distribution. The product under the ligand/Cr ratio of 0.5 obeys molecular weight with Schulz–Flory distribution [144]. Moreover, odd numbered 1-olefin is observed at sub-stoichiometric ratios. These results are accounted for by the formation of binuclear complex (PNP)Cr₂ or higher aggregates and/or chromium carbine complex. Scheme 5.12 shows a proposed mechanism for selective ethylene tetramerization.

Scheme 5.12

Proposed mechanism involving binuclear Cr complex for the Cr-catalyzed ethylene tetramerization

It involves conversion of dinuclear metallacyclopentane intermediates to a large dinuclear metallacycle species, which causes selective formation of 1-octene [145]. Two chromacyclopentanes in the catalyst molecule undergo 1,2-reductive elimination to give the dinuclear intermediate, having 1,8-octanediyl ligand. Elimination of 1-octene regenerates the dichromium catalyst precursor.

5.5 Dimerization of α-Olefins

Oligomerization of α-olefins produces linear and/or branched products depending on the regiochemistry of the monomer insertion to the metal-carbon bond. Scheme 5.13 summarizes relationship between regiochemistry and structure of the product formed by 1-hexene dimerization via Cossee mechanism. First 1,2-insertion of 1-hexene to metal-hydride bond, followed by 2,1-insertion of the monomer and β-hydrogen elimination leads to the linear product, whereas the successive 1,2-insertion of the two monomer molecules yields the branched product with a vinylidene group. Generally the products contain vinylene (inner olefin) and/or vinylidene (exo-methylene) groups. Studies on the dimerization of α-olefins were reported by several groups.

Scheme 5.13

Relationship between regiochemistry and structure of the product in 1-hexene dimerization

Reaction of ethylene and α-olefins by using zirconocene/MAO catalysts generally produces the copolymers. On the other hand, Bergman reported formation of the selective dimeric products (80–90 % yield) in the reaction under low Al/Zr ratio (ca. 1:1) [146]. The produced codimer contains exo-methylene group, and formed via double 1,2-insertion of the α-olefin into the Zr–H and Zr–C bond. The presence of Cl ligand rather than alkyl ligand at the zirconium center is proposed, which makes β-elimination easier than further insertion of olefin.

Ni complex with 1,3-diketonate ligand (Chart 5.11i), which has been known as the common catalyst for ethylene oligomerization, promotes dimerization of α-olefins in the absence of cocatalyst to give linear dimer predominantly (62–85 %) [147]. Higher olefins show decreased reactivity (128–770 mol (mol Ni)⁻¹ h⁻¹). Preferential formation of the linear dimer indicates the reaction proceeds via 1,2-insertion of the first olefin to Ni–H bond followed by 2,1-insertion of the second to the Ni–C bond. Although the catalyst loses its activity at room temperature, it can be stored as toluene solution at 0 °C without significant decomposition for over 30 days [148]. Activity of the Ni catalyst is improved by using it (up to TOF = 2,100 mol (mol Ni)⁻¹ h⁻¹) in the buffered ionic liquid [149]. High selectivity for dimerization (98 %) and linearity (64 %) is maintained. Biphasic reaction medium also allows easy catalyst recovery and recycling. Diimine Ni complexes, which are active for ethylene oligomerization, promote dimerization of propylene. The products are a mixture of linear and branched olefins, as a result of 1,2- and 2,1-insertion of the monomer into Ni–H and/or Ni–C bond.

Chart 5.11

The catalysts for linear dimerization of α-olefins

Small reported the iron and cobalt complexes with bis(imino)pyridine ligands are effective for linear dimerization [150] (Chart 5.11ii). The complexes with bulky substituents at the ligands show high activity for ethylene polymerization. The iron complex also promotes polymerization of propylene, which proceeds via 2,1-insertion of the monomer to the Fe–C bond. In contrast, bis(imino)pyridine iron complexes with less sterically hindered aryl groups are effective for oligomerization of ethylene (Sect. 5.2.2 of this chapter and Chart 5.2ii) and head-to-head dimerization of various α-olefins such as propylene, 1-hexene, and 1-decene to afford the corresponding linear dimers as main product (up to 80 %). The mechanism of the reaction involves selective 1,2-insertion of α-olefin into the iron-hydride bond and 2,1-insertion of the olefin into the metal-carbon bond, followed by chain transfer via β-hydride elimination. The methyl-branched dimers also form as a result of successive 2,1-insertion of the α-olefin.

The bis(imino)pyridine cobalt complexes are also active for the dimerization of α-olefins [151]. Although the activity is lower than the corresponding iron complexes, linearity of the product is higher (>97 %). In the reaction of 1-butene, 1-hexene, and 1-decene, the dimeric products are formed in high selectivity (>82 %). The reaction mechanism is similar to Fe-catalyzed linear dimerization of α-olefins. In contrast, the reaction of propylene produces not only linear dimer, but also trimers, teramers, and pentamers. Interestingly, all of the oligomers are of high linearity (>93.5 %). Formation of such linear higher olefins is accounted for by the successive 2,1-insertion of propylene and 1,2-shift of the Co center (chain growth pathway) and/or the linear codimerization of once-formed 1-hexene (step growth pathway).

Gibson also reported the bis(imino)pyridine Co catalyst for the dimerization of 1-butene and 1-hexene to give the corresponding linear dimer with internal olefinic group (rich in E isomers) [152]. The reaction of propylene leads to 1-hexene without isomerized product, similarly to the results by Small. The formation of linear trimers and tetramers also form during the reaction, as a result of combined chain growth and step growth mechanism. The complexes with fluorinated aryl imino group show much higher catalytic activity than the nonfluorinated counterpart.

Although inner olefins are frequently formed in the dimerization of α-olefins, Broene recently reported that half cobaltocene complex with P(OMe)₃ ligand promotes dimerization of 1-hexene to give 1-decene as a byproduct [153] (Scheme 5.14). The main product is 2-butyl-1-octene, which is formed by double 1,2-insertion of 1-hexene to Co–H and Co–C bonds. In contrast, 1-decene is formed via 1,2-insertion of 1-hexene to Co–H and 2,1-insertion of the monomer to Co–C bond, followed by isomerization of secondary alkyl Co species to primary alkyl Co species. The isomerization is considered to proceed via β-hydrogen elimination and re-insertion, similar to the olefin polymerization catalyzed by Pd and Ni complexes with diimine ligands (chain walking).

Scheme 5.14

Proposed mechanism for the formation of 1-decene in Co-catalyzed dimerization of 1-hexene

The Ta-catalyzed dimerization of olefins was also reported by Schrock in 1980 [154]. The reaction proceeds via metallacycle mechanism, similar to the Ta-catalyzed trimerization of ethylene. The product is a mixture of tail-to-tail dimer and head-to-tail dimer. In the case of propylene, 1-pentene, and 1-octene the major product is the tail-to-tail dimer (86–98 %), whereas the reaction of 4,4-dimethyl-1-pentene results in selective formation of head-to-tail dimer (100 %). The activity of the catalyst is on the order of one turnover/h at 30 °C.

5.6 Trimerization of α-Olefins

Although many reports on ethylene trimerization have appeared in the literatures, examples of trimerization of α-olefins are very limited. In 2000, Köhn reported that Cr complexes with triazacyclohexane ligand in combination with MAO promote trimerization of α-olefins [155, 156]. The product is a mixture of the isomers of α-olefin trimers. At room temperature, the trimerization stops after a few hours (about 1,000 turnovers) caused by decomposition of the catalyst, while the catalyst keeps its activity for several days at 0 °C (conversion reaches >90 %).

The plausible reaction mechanism involves formation of cationic Cr(I) species and formation of metallacycle intermediate by oxidative cyclization, similar to the ethylene trimerization (Scheme 5.15). The rate-determining step is insertion of the third olefin to Cr–C bond, which is followed by β-hydrogen elimination and reductive elimination of the α-olefin trimers. The activity and selectivity of the catalysts are largely affected by the alkyl group of the ligand. Branched structure in the α-position of the alkyl group increases isomerization rather than trimerization, and decreases activity of the catalyst.

Scheme 5.15

Codimerization of ethylene with α-olefins

5.7 Cooligomerization of Ethylene and α-Olefins

In principle, reaction of ethylene and α-olefins can afford the corresponding cooligomer, but the actual examples are very limited. Cr complex with bis(carbine)pyridine ligand promotes codimerization of ethylene with 1-octene via metallacycle mechanism [128] (Scheme 5.15). The product contains not only 2-ethyloct-1-ene, formed by the reaction of ethylene with 1-octene, but also C12, C14, and C18 products (cotrimerization and cotetramerization product). Hessen also reported that half titanocene complex promote cotrimerization of ethylene with 1-hexene to give C10 products (5-methylnon-1-ene, mainly) [111] (Scheme 5.16).

Scheme 5.16

Ru-catalyzed codimerization of vinyl monomers

5.8 Cooligomerization of Other Vinyl Monomers

Codimerization of ethylene with various vinyl monomers (hydrovinylation) has been known [157, 158]. The catalyst for the hydrovinylation involves Ni, Pd, and Ru. Examples of asymmetric hydrovinylation have been also reported [159]. Recently, Kondo revealed that ruthenium catalysts catalyze codimerization of different vinyl monomers in very high selectivity [160]. For example, RuCl₃(tpy)/Zn (tpy = 2,2′:6′2′′-terpyridine) promote codimerization of norbornene with acrylates [161]. Ru(cod)(cot) is effective for codimerization of dihydrofuran and N-vinylacrylamide with acrylates (Scheme 5.17) [162]. Codimerization of ethylene with styrene catalyzed by Ru(cod)(cot) yields 1-phenyl-1-butene [163], which is in contrast to that 3-phenyl-1-butene is obtained in the general hydrovinylation.

Scheme 5.17

.

5.9 Summary

In contrast to transition metal-catalyzed olefin polymerization, the development of oligomerization catalysts is much less matured. In this decade, novel highly active and selective ethylene oligomerization catalysts have been disclosed, which enabled selective formation of 1-hexene and 1-octene from ethylene in high catalytic activity. There are still rooms for investigation in selective synthesis of higher α-olefins by ethylene oligomerization and selective oligomerization of α-olefins. Further development of novel oligomerization catalysts is expected.